1. Field of the Invention
The present invention relates to pseudospark switches and their applications as electron beam sources for free electron lasers, high power magnetrons, and compact x-ray generators; high power switches for laser and microwave HV units, capacitor discharges, and crow bar protection; and materials processing applications such as cutting, drilling, and film deposition.
2. Description of Related Art
Pseudospark discharge e-beam generators and preionization-controlled open-ended hollow cathode (PCOHC) transient discharge e-beam sources are described in references 1-8 and U.S. Pat. Nos. 5,055,748; 5,126,638; 5,502,356; 5,850,125; and 6,104,022, incorporated by reference herein in their entirety.
Pseudospark switches operate on the left branch of the Paschen curve, close to vacuum breakdown. In these devices, the inter-electrode distance, d is comparable to or smaller than the electron mean free path for collision, λmfp at the nominal operating pressure, d≦λmfp. This configuration prevents avalanche ionization and breakdown. The anode and cathode apertures play a key role in the operation of the device by increasing the effective path for ionization and breakdown at lower voltages. When breakdown occurs, the discharge is confined on the axis of the device, and little or no erosion of the electrodes takes place. A classic pseudospark discharge starts with the ignition of the high voltage glow discharge along the axis of the device. During this period, high-energy ions are accelerated into the hollow cathode where they produce secondary electrons. These electrons are accelerated toward the cathode aperture by the potential difference and then are extracted toward the anode. The remaining low energy ions modify the potential distribution in the hollow cathode chamber (HCC) and retard the movement of the highly energetic ions from the main gap.
The accumulation of positive charge inside the HCC forms a virtual anode. Thus, the hollow cathode and the main chamber are separated by a potential barrier. The potential barrier will allow only sufficiently energized electrons to leave, decreasing the electron current. The subsequent growth of the anode plasma and the neutralization of the virtual anode will eventually allow the low energy electrons to escape the HCC and cause a steep rise in the e-beam current. The result of this two-step process is an e-beam with two current peaks (FIG. 1). The first peak (I) is relatively weak and is due to highly energetic electrons formed prior to gap breakdown. The second, larger peak (II) is due to less energetic (bulk) electrons formed during the collapse of the anode-cathode field. During its propagation, the low energy e-beam of the classic pseudospark is scattering and degrading due to frequent electron-neutral particle collisions.
Electron beam quality is measured by two related quantities: beam emittance and beam brightness. The emittance refers to the collimation of the beam and is defined as the product between the beam radius and the angle of the velocity vector with the symmetry axis. Beam brightness is directly proportional to beam current and inversely proportional to the square of the emittance, and measures beam intensity and beam collimation. Electron-beams produced by existing pseudospark switches, in qualitative terms, have wider energy distributions (lower brightness) and higher divergence (higher emittance) that desired for some applications. For example, the use of pseudospark discharge e-beam generators for free electron lasers (FELs) has been prevented by the axial velocity-spread of the beam. Typically the maximum energy spread for either the high-gain Compton or collective Raman regimes must be less than 1% of total beam energy for efficient transfer of electron energy to electromagnetic waves. FIG. 1 shows an exemplary expected e-beam current produced by the present invention (AMPS).